"In the Fourth Year of the War" by Harlan Ellison

I've said this before here, but let me reiterate: I pity every single person that has ever pissed off Harlan Ellison (I believe that they are legion). "In the Fourth Year of the War" is another sort of revenge story in which the main character, now in the fourth year of a war with the homicidal split personality in his head, is driven to murder every person who has done him or his loved ones wrong, including an old neighbour from his childhood that had his dog put down, and even his ex-wife.

The story itself seems to hinge on the idea that we are made up of our memories, including the bad ones, and that, as human beings, we never really let things go (quite disturbingly, Ellison mentions in the introduction from the Shatterday anthology that one of the inspirations for this story was a woman who had his dog put down when he was young, whom he never forgave). Of course, he's not condoning the main characters murderous actions in the story, rather that this is what not properly dealing with the memories we carry with us can lead to, and that, in some way, all people have a similar problem to the protagonist, which only serves to make the story that much creepier.